1. Field of the Invention
The present invention relates to a focusing state determining adapter for a taking lens, and more particularly, to a focusing state determining adapter for a taking lens which is applied for determining a focusing state in autofocus control of the taking lens.
2. Description of the Related Art
As an autofocus method in video cameras, a contrast method is used where the best focus is sought so that contrast of images formed on the imaging element becomes the largest. The problem of the contrast method is that the focusing operation is slow, since a hill-climbing method is adopted where the best focus at which the contrast is the largest is achieved while the focus lens is being moved.
Japanese Patent Application Publication No. 55-76312 and Japanese Patent Publication No. 7-60211 disclose a method for determining a focusing state (i.e., a front focusing state, a rear focusing state or a just focusing state) of the taking lens by using a plurality of imaging elements to which optical path lengths from the taking lens are different from each other. For example, two imaging elements for determining a focusing state (hereinafter referred to as “focus-imaging elements”), which take images of the same visual fields with another imaging element that takes an image for picturing (hereinafter referred to as a “picture-imaging element”), are arranged at positions to which optical path lengths from the taking lens are longer and shorter, respectively, than the optical path length from the taking lens to the picture-imaging element. High frequency components are extracted from signals of the images taken by the focus-imaging elements. Then, focus evaluation values, which indicate focusing degree (contrast) with respect to the imaging faces of the focus-imaging elements, are obtained according to the extracted high frequency components. The obtained focus evaluation values are then compared to each other, so that it is determined that the focusing state with respect to the imaging face of the picture-imaging element is a front focusing state, a rear focusing state, or a just focusing state. Thus, the focus determination for the autofocus control can be made. Applying this focusing state determining method has an advantage in that the focusing operation can be fast, since it can be determined not only whether the focusing state is the just focus, but also whether the focusing state is the front focus or the rear focus when not the just focus.
As high-definition television broadcasting has become generalized in recent years, the resolving power of the taking lenses and the cameras that are used in the television camera system has become higher. In the actual taking of pictures for the high-definition television with manual focus, it is difficult to achieve the best focus, since the operator focuses the taking lens while referring to pictures in a viewfinder with his or her own eye and there are limits of the resolving power of the viewfinder and eye. Even if the operator has determined that an image is in the just focus while taking, in some cases the image is discovered to be out of focus while being replayed on a large-sized monitor. Therefore, the autofocus function becomes necessary for focusing the taking lens more accurately than the manual focus.
However, the general television camera does not have the autofocus function, and it is not practical that the autofocus function is provided to all cameras in future, since not all the operators use the autofocus function. Therefore, it is considered that the cases will increase that the autofocus function cannot be used in spite of eager for using, in the camera systems already on sale and the camera systems on sale in future.